Hematopoiesis and leukemogenesis are developmental phenomena, in which complex interactions of multiple genes give rise to orderly differentiation or to maturation arrest and uncontrolled proliferation. While many of the terminal events in these pathways have been elucidated by cellular, biochemical and transgenic mouse studies, the genes responsible for the most fundamental developmental decisions in hematopoiesis and leukemogenesis have been less well-defined. Tumor suppressor genes (TSGs), first identified in hereditary human cancers, are likely to be among this group of fundamental genes. TSGs are key elements of cellular pathways that regulate growth and differentiation. Specific TSGs have been shown to be mutated or deleted in primary human leukemias and leukemic cell lines. Mouse knock-out models and patients deficient in several of these genes demonstrate defective hematopoiesis or develop leukemias. To better understand the function of tumor suppressor genes in normal hematopoiesis and leukemogenesis we have chosen as a model the zebrafish (Danio rerio), a vertebrate system that combines physiology and development with powerful genetics. We plan to create zebrafish strains deficient in known TSGs and to characterize hematopoietic defects and the development of leukemia in these strains. A suppressor or enhancer screen for mutations that correct the defects or cure the leukemia will lead to the isolation of additional genes in these pathways. The experiments described in this proposal are likely to identify novel proteins that play key roles in the control of hematopoiesis and the genesis of leukemia. These proteins will be potential therapeutic targets for the treatment of aplastic anemia, myelodysplasia and leukemia in humans.